There have been many attempts to develop compact, high precision, low tolerance narrow band optical filters centered at predetermined wavelengths for application in areas such as spectroscopy, optical networks and optical links, and more particularly optical communication systems. Optical filters are some of the most ubiquitous of all passive optical components found in most optical communication systems. One use of optical filters is in the field of optical communications where only a signal of a predetermined wavelength is to be passed.
Although this invention is particularly useful with narrow band optical filters, it is not limited to filters which pass only a very narrow band of light (e.g. .+-.0.25 nanometers or less) centered at a predetermined wavelength; however, since such filters are extremely difficult to make and consequently relatively expensive to manufacture the applicability of this invention for the narrow band is evident. Thus, re-using a filter of this type in accordance with the teachings of this invention has significant advantages. One known means for providing a selective, narrow band, optical filter, is by utilizing a wavelength selective interference filter element whose wavelength characteristic depends on the angle of incidence. Thus, by varying the angle of light incident upon the interference filter, the wavelength of the light that is passed by the filter varies. Such a filter element is described in U.S. Pat. No. 5,331,651 issued Jul. 19, 1994 and assigned to the Hewlett-Packard Company. Another filter of this type is described in allowed U.S. patent application Ser. No. 08/442,365 in the name of Si assigned to JDS Fitel Inc. of Canada.
Since these high precision, low tolerance narrow band optical filters centered at predetermined wavelengths are very difficult and costly to make, it is an object of this invention to pass multiple beams through a small filter element a plurality of times in order to attempt to increase the usage of the filter or at least provide a more economical filtering system.
An optical coupler is a device that is used to transfer optical signals from at least one waveguide to another. An optical combiner is a device that combines more than one input signal into a single waveguide. The instant invention hereafter described applies to both couplers and combiners. Hence the term coupler is meant to encompass both types of devices and is meant to be interpreted broadly, to cover both terms.
A wavelength division multiplexing/demultiplexing (WDM) coupler transfers input optical signals from a plurality of input information channels to a plurality of output information channels in response to the wavelength of the input signals. A goal of any WDM coupler is that the crosstalk between channels is zero, i.e., that an untargeted output channel is effectively isolated from the signals on a targeted output channel, so that none of the output channels leak onto the untargeted channel.
FIG. 1 is a prior art representational block diagram of a WDM optical receiver system for separating an incoming optical signal as disclosed by Pan et al. and assigned to E-Tek Dynamics, Inc. The system comprises wavelengths of light .lambda..sub.1,2,3,4,5,6,7,8 into eight separate channels. Although this device may achieve some channel separation, the single filtering of a particular optical signal in a cascaded filtering device has certain drawbacks. For example, in FIG. 1, the lensed optical filtering device 271 is designed to reflect light of wavelengths .lambda..sub.1,2,3,4, and to pass light of wavelengths .lambda..sub.5,6,7,8. However, with dichroic filters of this type, it has been found that a small percentage of signal light of wavelength .lambda..sub.1,2,3,4 is not reflected and passes through conventional filters such as 271, thereby not providing enough isolation between channels to meet certain requirements. This problem becomes exacerbated when subsequent filtering is not provided to eliminate small leakage of unwanted wavelengths. For example, in FIG. 1, when perfect separation of wavelengths .lambda.1 and .lambda.2 is not provided by filter 274, some light of wavelength .lambda.1 will leak into the separated light of wavelength .lambda.2. Conversely some light of wavelength .lambda.2 will leak into the separated light of wavelength .lambda.1. Hence, a filtering arrangement is often preferred to achieve greater isolation between all or certain channels.
It is an object of this invention, to provide an optical system, wherein a single optical filter is used in a manner, which provides repeated filtering at least a same wavelength of an optical signal.
It is a further object of this invention to provide a less expensive coupler system that obviates some of the limitations of the prior art.